Control & Coordination in MAMMALS Flashcards
what is a hormone?
is a chemical substance produced by an endocrine gland and carried by the blood
What does a hormone (chemical) transmit?
- what does it alter?
They are chemicals which transmit information from one part of the organism to another and bring about a change
They alter the activity of one or more specific target organs
What are hormones used to control?
functions that do not need instant responses
What does the endocrine gland produce? What is it collectively known as?
The endocrine glands that produce hormones in animals are known collectively as the endocrine system
What is a gland?
is a group of cells that produces and releases one or more substances (a process known as secretion)
What type of hormones are released into the blood?
insulin, glucagon, ADH and adrenaline are cell-signalling molecules
Why do Endocrine glands have a good blood supply?
as when they make hormones they need to get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about a response
What type of cells do hormones only affect?
only affect cells with receptors that the hormone can bind to
- These are either found on the cell surface membrane, or inside cells
- Receptors have to be complementary to hormones for there to be an effect
What type of hormones are peptides or small proteins?
insulin, glucagon and ADH
insulin, glucagon and ADH
- They are water-soluble and so cannot cross the phospholipid bilayer of cell surface membrane
- These hormones bind to receptors on the cell surface membranes of their target cells, which activates second messengers to transfer the signal throughout the cytoplasm
What type of hormones are steroid hormones?
testosterone, oestrogen and progesterone
testosterone, oestrogen and progesterone
They are lipid-soluble and so can cross the phospholipid bilayer
These hormones bind to receptors in the cytoplasm or nucleus of their target cells
The human nervous system consists of the:
Central nervous system (CNS) – the brain and the spinal cord
Peripheral nervous system (PNS) – all of the nerves in the body
What does the Central nervous system (CNS) include?
the brain and the spinal cord
What does the Peripheral nervous system (PNS) include?
all of the nerves in the body
What does the nervous system allow us to make sense of?
our surroundings and respond to them and to coordinate and regulate body functions
How is info sent through the NS?
As nerve impulses
What is a nerve impulse?
electrical signals that pass along nerve cells known as neurones
What is a nerve impulse?
electrical signals that pass along nerve cells known as neurones
What is a nerve cell known as?
Neurone
What is a bundle of neurones known as ?
nerve
What do neurones coordinate?
print out the table
the activities of sensory receptors (eg. those in the eye), decision-making centres in the central nervous system, and effectors such as muscles and glands
Neurones have a long fibre. What is it known as?
An Axon
What is the axon insulated by?
by a fatty sheath with small uninsulated sections along its length (called nodes of Ranvier)
what is the axons sheath made of ?
myelin, a substance made by specialised cells known as Schwann cells
On the axon their cell bodies contain many extensions called?
dendrites
This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
When is Myelin made?
What does this mean?
when Schwann cells wrap themselves around the axon along its length
This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next
This means that less time is wasted transferring the impulse from one cell to another
What are the 3 main types of neurones?
sensory, relay and motor
Sensory neurones
carry impulses from receptors to the CNS (brain or spinal cord)
Relay (intermediate) neurones
are found entirely within the CNS and connect sensory and motor neurones
Motor neurones
carry impulses from the CNS to effectors (muscles or glands)
Motor neurones have:
A large cell body at one end, that lies within the spinal cord or brain
A nucleus that is always in its cell body
Many highly-branched dendrites that extend from the cell body, providing a large surface area for the axon terminals of other neurones
Sensory neurones have the same basic structure as motor neurones, but have
A cell body that branches off in the middle of the cell - it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion
What do Sensory neurones, relay (intermediate)?
neurones and motor neurones work together to bring about a response to a stimulus
What is a reflex arc?
a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
What does the reflec arc not involve?
the brain
As a reflex arc does not involve the brain, a reflex response is..
quicker than any other type of nervous response
Examples of simple reflex actions that are coordinated by these pathways are:
Removing the hand rapidly from a sharp or hot object
Blinking
Focusing of the eye on an object
Controlling how much light enters the eye
In the example above:
A pin (the stimulus) is detected by a pain receptor in the skin
The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
Electrical impulses are passed on to relay neurone in the spinal cord
The relay neurone connects to the motor neurone and passes the impulses on
The motor neurone carries the impulses to the muscle in the leg (the effector)
The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)
define receptor cell
A cell that responds to a stimulus
What are receptors?
transducers
define transducers
they convert energy in one form (such as light, heat or sount) into energy in an electrical impulse within a sensory neurone
Where are receptor cells often found?
in sense organs (eg. light receptor cells are found in the eye)
Why are some receptors specialised cells?
for eg:
- light recpetors in the eye and chemoreceptors in taste buds
- they delect a specific types of stimulis and influence the electrical activity of a sensory neurone
Other receptors, such as some kinds of touch receptors, are just the ends of the sensory neurones themselves
When receptors are stimulated they are?
depolarised
Receptor cells
if a stimulus is very weak. What happens to the cells?
the cells are not sufficiently depolarised and the sensory neurone is not activated to send impulses
Receptor cells
if the stimulus is strong enough what is activated?
the sensory neurone is activated and transmits impusles to the CNS
What is the surface of the tongue covered in ?
many small bumps -> papillae
what is the surface of each papilla covered in?
Many taste buds
what does each taste bud contain?
many receptor cells -> chemoreceptors
What are chemoreceptors sensitive to?
chemicals in food and drinks
What is each chemoreceptor covered with?
receptor protiens
What do different receptor proteins detect?
different chemicals
what do the chemoreceptors in the taste buds detect?
salt (NaCl) respond directly to Na ions
An example of the sequence of events that results in an action potential in a sensory neurone
If salt is present in the food (dissolved in saliva) being eaten or the liquid being drunk:
- Na ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptors
- leads to depolarisation of the chemoreceptor cell membrane
- increase in positive charge inside cells -> the receptor potential
- if there is sufficient stimulation by Na ions and sufficient depolarisation of membrane, the receptor potential becomes large enough to stimulate voltage-gated calcium channel proteins to open,
- Ca ions enter the cytoplasm of chemoreceptor cell and stimulate exocytosis of vesicles containing neurotransmitter from the basal membrane of chemoreceptor
- neurotransmitter stimulates action potential in sensory neurone
- sensory neurone then transmits an impulse to brain
If the stimulus is very weak or below a certain threshold, ..
the receptor cells won’t be sufficiently depolarised and the sensory neurone will not be activated to send impulses
If the stimulus is strong enough to increase the receptor potential above the threshold potential …
then the receptor will stimulate the sensory neurone to send impulses
This is an example of the all-or-nothing principle
An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
Rather than staying constant, threshold levels in receptors
often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones
What do neurones transmit?
electrical impulses, which travel extremely quickly along hte neurone cell surface membrane from one end of the neurone to the other
electrical impulses transmitted from neurones are not a flow of electrons, what are they known as?
Action potentials
How do action potentials occur?
very breif change in the distribution of elecrical charge across the cell surface membrane
what are action potentials caused by?
rapid movement of Na and K ions across the membrnae of the axon
what is a resting axon?
one that is not transmitting impulses
in a resting axon:
what does the inside of the axon always have?
a slightly negative electrical potential compared to outside the axon
what is the PD of resting axon?
about -70mV (ie. the inside of the axon has an electrical potential about 70mV lower than the outside)
what is the PD of resting axon?
about -70mV (ie. the inside of the axon has an electrical potential about 70mV lower than the outside)
what is resting potential?
the difference in electrical potential that is maintained across the CSM of a neurone when it is not transmitting an action potential; it is normally about -70mV inside and is partially maintained by Na-K pumps.
what is PD?
the difference in electrical potential between 2 points’ in the NS, between the inside and the outside of a CSM such as the membrane that encloses the axon
NS- nervous system CSM- cell surface membrane
Several factors contribute to maintaining the resting potential:
-
Na-K pumps in the axon membrane:
these pumps move Na+ ions out of the axon and K+ ions into the axon. the pump proteins uses energy from the hydrolysis of ATP to continue moving these ions against their conc grad -
many large, negatively charged molecules (anions) inside the axon:
this attracts K+ ion , reducing the chance of them diffusing out of the axon -
impermeability of the axon membrane to ions:
Na ions cannot diffuse through the axon membrane when the neurone is at rest -
closure of voltage-gated channel proteins (required for axion potentials in the axon membrane):
stops Na + K ions diffusing through the axon membrane
what type of proteins in the axon membrane allow Na/K ions to pass through?
channel proteins
what are voltage-gated channel proteins?
they open and close depending on the electrical potential (or voltage) across the axon membrane
when are voltage-gated channel protens closed?
when the axon memebrane is at its resting potential
When an action potential is stimulated (eg. by a receptor cell) in a neurone, the following steps occur:
- Na channel proteins in axon membrane open
- Na+ ions pass into axon down electrochemical grad (there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
- reduces PD across axon membrane as inside of axon becomes less negative -> depolarisation
- triggers voltage gated Na channels to open, allowing more Na+ ions to enter causing more depolarisation
- This is an example of positive feedback (a small initial depolarisation leads to greater and greater levels of depolarisation)
- if PD reached around -50mV (threshold value), many more channels open adn more Na ions enter causing inside of axon to reach a potential around +30mV
- action potential generated
what does the depolarisation of the membrane at the site of the first action potential cause?
current to flow to the next section of the axon membrane, depolarising it and causing Na+ ion voltage-gated channel proteins to open
The depolarisation of the membrane at the site of the first action potential causes current to flow to the next section of the axon membrane
what is the ‘flow’ of current caused by?
the diffusion of Na+ ions along the axon from an area of high conc to an area of low conc
- this triggers the production of another action potential in this section of the axon membrane and the process continues
- in the bodt rhis allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane
Very shortly (about 1 ms) after an action potential in a section of axon membrane is generated,…
all the Na ion voltage-gated channel proteins in this section close, stopping any further Na+ ions diffusing into the axon
- K ion voltage-gated channel protiens in this section of axon membrane now open, down their conc grad
- this returns PD to normal (about -70mV) - > process known as repolarisation
there is a short period og hyperpolarisation.
this is when?..
the PD across this section of axon membrane becomes more negative than the normal resting potential
- K ion voltage-gated channel proteins then close and the Na+ ion channel proteins in this section of membrane become responsive to depolarisation again
- until this occurs, this section of the membrane is in a period of recovery and is unresponsive -> known as refractory period
what deos the speed of conduction of an impulse refer to?
how quickly the impulse is transmitted along a neurone
the speed of conduction of an impulse is determined by 2 factors.
What are they?
- the presence or absence of myelin (ie. whether or not the axon is insulated by a myelin sheath)
- the diameter of the axon
in unmyelinated neurones what is the speed of conduction?
very slow
by insulated the axon membrane, what does the presence of myelin increase?
increases the speed at which action potentials can travel along the neurone
Transmission of an action potential in a myelinated axon by saltatory conduction
- In sections of the axon that are surrounded by a myelin sheath, depolarisation (and the action potentials that this would lead to) cannot occur, as the myelin sheath stops the diffusion of sodium ions and potassium ions
- Action potentials can only occur at the nodes of Ranvier (small uninsulated sections of the axon)
- The local circuits of current that trigger depolarisation in the next section of the axon membrane exist between the nodes of Ranvier
- This means the action potentials ‘jump’ from one node to the next
- This allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter
why is the speed of conduction of an impulse along neurones with thicker axons greater than along those with thinner ones?
thicker axons have an axon membrane with a greater SA over which diffusion can occur
- this increases the rate of diffusion of Na and K ions, which in turn increases the rate at which depolarisation and action potentials can occur
what occurs very shortly (about 1 ms) after an action potential has been generated in a section of the axon membrane ?
- all the Na ion voltage-gated channel proteins in this section close
- stops any further Na ions from diffusing into the axon
- K ion voltage-gated channel proteins open, allowing diffusion of K ions out of axon down thier conc grad
- gradualy returns PD to normal (about -70mV) – a process known as repolarisation
- once resting potential is close to being reestablished, the K ion voltage-gated channel proteins close and Na ion channel protiens in this section of the membrane become responsive to depolarisation agian
- until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive -> known as refractory period
what is a refractory period?
A period of time during which a neurone is recvoring from an action potential, ansd during which another action potential cannot be generated.
The refractory period is important for the following reasons:
- ensures that action potentials are discrete events, stopping them from merging into one another
- it ensures that ‘new’ action potentials are generated ahead, rather than behind the original ction potential that has just occured
- this means that the impulse can only travel in 1 direction, which is essential for the successful and efficient transmission of nerve impulses along neurones
- means there is a min time between action potentials occuring at any one place along a neurone
- the length of the refractory period is key in determining the maximum freq at which impulses can be transmitted along neurones (between 500 and 1000 per second)
When 2 neurones meet, they do not actually come into physical contact with each other, a very small gap known as?
the synaptic cleft, separates them
what do the ends of 2 neurones, along with the synaptic cleft form?
a synapse
Synaptic transmission – basic mechanism
- electrical impulses cannot ‘jump’ across synapses
- when an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles at the presynaptic membrane
- the neurotransmitters diffuse across the synaptic cleft and temp bind with receptor molecules on the pstsynaptic membrane
- this stimulates th epostsynaptic neurone to generate an electrical impulse that then travels down the axon of the postsynaptic neurone
- the neurotransmitters are then destroyed or recycles to prevent continued stimulation of the seocn neurone, which could cuase repreated impulses to be sent
What is one of the key neurotransmitters used throughout the nervous system?
acetylcholine (ACh)
What are the synapses that use the neurotransmitter ACh known as?
cholinergic synapses
Synaptic transmission using acetylcholine (ACh)
- Action potential arrives, depolarising presynaptic membrane
- Ca ion channels proteins open, Ca ions diffuse in
- presynaptic vesicles fuse with membrane
- ACh released
- ACh diffuses across synaptic cleft
- ACh binds to receptor proteins
- receptor proteins open, Na ions diffuse through
- Postsynaptic membrane is depolarised
- ACh broken down into Acetate and Choliene by Acetylcholinesterase
- choliene recycled into ACh
what does The enzyme acetylcholinesterase catalyse ?
the hydrolysis of the ACh molecules into acetate and choline
The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives
This entire sequence of events takes 5 – 10 ms
How the myofibrils within muscle fibres are stimulated to contract
- Action potential
- Ca ions diffuse into neurone
- ACh- containing vesicles fuse with presynaptic membrane
- ACh diffuses across neuromuscular junction
- ACh binds to sarcolemma receptor protein s
- Na ions diffuse into sarcolemma
- Action potential passes along sarcolemma and down T- tubules
- Ca ions diffuse out of sarcoplasmic reticulum (SR) and into sarcoplasm
- Ca ions bind with troponin molecules, starting the process of muscle contraction
What type of muscle makes up in the body that are attached to the skeleton?
striated muscle
what is a striated muscle made up of?
muscle fibres
A muscle fibre is a highly specialised cell-like unit:
Each muscle fibre contains an organised arrangement of contractile proteins in the cytoplasm
Each muscle fibre is surrounded by a cell surface membrane
Each muscle fibre contains many nuclei – this is why muscle fibres are not usually referred to as cells
The different parts of a muscle fibre have different names to the equivalent parts of a normal cell:
Cell surface membrane = sarcolemma
Cytoplasm = sarcoplasm
Endoplasmic reticulum = sarcoplasmic reticulum (SR)
The sarcolemma has many deep tube-like projections that fold in from its outer surface:
These are known as transverse system tubules or T-tubules
These run close to the SR
The sarcoplasm contains mitochondria and myofibrils
The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction
what do the membranes of the SR contain ?
protein pumps that transport calcium ions into the lumen of the SR
where are the myofibrils located?
in the sarcoplasm
Each myofibril is made up of two types of protein filament:
Thick filaments made of myosin
Thin filaments made of actin
These two types of filament are arranged in a particular order, creating different types of bands and line
Parts of myofibril
H band - only thick myosin filaments present
I band - only thick actin filaments present
A band - contains areas wheere only myosin filaments are present and areas where myosin and actin filaments overlap
M line - attachment for myosin filaments
Z line - Attachment for actin filaments
Sarcomere - the section of myofibirl between 2 Z lines
The thick filaments within a myofibril are made up of myosin molecules
These are fibrous protein molecules with a globular head
The fibrous part of the myosin molecule anchors the molecule into the thick filament
In the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line
The thin filaments within a myofibril are made up of actin molecules
These are globular protein molecules
Many actin molecules link together to form a chain
Two actin chains twist together to form one thin filament
A fibrous protein known as tropomyosin is twisted around the two actin chains
Another protein known as troponin is attached to the actin chains at regular intervals
how do muscles cause movement ?
by contracting
during muscle contraction..
sarcomeres within myofibrils shorten as the Z discs are pulled closer together
This is known as the sliding filament model of muscle contraction
How muscles contract – the sliding filament model
An action potential arrives at the neuromuscular junction
Calcium ions are released from the sarcoplasmic reticulum (SR)
Calcium ions bind to troponin molecules, stimulating them to change shape
This causes troponin and tropomyosin proteins to change position on the actin (thin) filaments
Myosin binding sites are exposed on the actin molecules
The globular heads of the myosin molecules bind with these sites, forming cross-bridges between the two types of filament
The myosin heads move and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
ATP hydrolysis occurs at the myosin heads, providing the energy required for the myosin heads to release the actin filaments
The myosin heads move back to their original positions and bind to new binding sites on the actin filaments, closer to the Z disc
The myosin heads move again, pulling the actin filaments even closer the centre of the sarcomere, causing the sarcomere to shorten once more and pulling the Z discs closer together
The myosin heads hydrolyse ATP once more in order to detach again
As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP, this process repeats until the muscle is fully contracted
